Nine government watchdog groups today called on the 2012 presidential candidates to lift the veil of secrecy that shrouds their biggest fundraisers, the so-called "bundlers" who use their connections to steer millions of dollars from well-heeled donors to the campaigns of their choice.

In letters sent to President Obama and Republican candidates Newt Gingrich, Ron Paul, Mitt Romney and Rick Santorum, the organizations asked that the campaigns disclose specific information about their major bundlers, identifying them by name and stating the precise amounts they raise.

"We are gearing up for the height of the 2012 election season and voters shouldn't be left in the dark about the powerful individuals who are helping the candidates raise money for their campaigns," said Gabriela Schneider, communications Director for the Sunlight Foundation, one of the watchdog groups. The other organizations that signed the letters include the Center for Responsive Politics, Common Cause, Campaign Finance Institute, the Campaign Legal Center, Democracy 21, the League of Women Voters, Public Citizen, and the U.S. Public Interest Research Group.

Bundlers are usually wealthy and well-connected people who raise money in excess of what an individual is legally allowed to donate -- $2,500 for the primary election and $2,500 for the general election -- by "bundling" checks from friends and associates in amounts that can reach $500,000.

Although federal law only requires campaigns to disclose the names of their bundlers who are registered lobbyists, former President George W. Bush began disclosing all bundlers who raised more than $100,000 during his 2000 presidential campaign -- a precedent that was followed by the major candidates of both parties in the 2004 and 2008 elections.

The good-government groups urged the candidates to begin releasing even more specific information about the individuals who bundle more than $50,000 for their campaigns, including the exact amount those bundlers raise, in their next financial disclosure.

Thus far, President Obama is the only 2012 presidential candidate to disclose voluntarily the names of his current campaign's bundlers beyond those who are registered lobbyists, and has also provided a general range that each has brought in. His campaign says 444 individuals have bundled contributions totaling over $74 million.

"While the President has led the way on transparency and reform -- disclosing his major volunteer fundraisers since 2007 and passing legislation that required lobbyists to make public if they are fundraising for a candidate -- Governor Romney, Sen. Santorum, and Speaker Gingrich are defying the precedent set by President Bush and Senator McCain and refusing to disclose their bundlers," Obama campaign spokesman Ben LaBolt told ABC News. "What do they have to hide from the American people?"

Romney, as required by law, has disclosed 16 lobbyists who have bundled close to $2.36 million to his campaign but has not released information on any other bundlers.

"We disclose all of the information about our donors as required by law and anyone who is interested can review it publicly," Romney spokesperson Andrea Saul told ABC News.

Saul also said that Obama "shouldn't be lecturing anyone on campaign finance."

"He broke his promise to live within the campaign spending limits, and is on track to be the first candidate to raise $1 billion for a political campaign," she said."

None of the other remaining Republican candidates have disclosed any bundled contributions, and their representatives did not immediately respond to a request for comment by ABC News.